It's true that if she were given the choice, she'd take them both in her arms and keep them with her, and watch them and love them and be with them until all time. If she had the choice, the paopu fruit would have been split into thirds to be shared equally among them.
But as much as it hurts to be left alone, it isn't that bad because they still include her, just not how she would choose to be included. They still talk to her, take her to dinner with them, invite her to their respective homes (though she can't imagine why they haven't simply moved in together), and do everything they can to make her their closest friend.
That just isn't what she wants.
But it's alright, because for all that she will never join them, she still has the right to see them like no one else does. She still gets to watch a spiky cinnamon hair splayed over a pale stomach as they lay together on the sandy beach. She still gets to see bright aqua eyes as they stare into deep, clear blue on top of the bent trunk of their tree as the sun sets in a blaze of color.
She doesn't get to be a part of it but...
If nothing else, Kairi is content to watch.